Security signalling with alarm identification

ABSTRACT

A security system is provided which includes a central monitoring system interconnected by a two-wire telephone circuit to one or more protection loops, each being composed of one or more series-connected reporting devices or stations. A DC power supply is located in the control station for generating a supervisory current flow between the control station and the protection loop, and each reporting device is composed of a signal generator in parallel with a low resistance shunt arranged to be broken by the detected occurrence of an event sought to be observed. Detection of the event interrupts the shunt to route the supervisory current through its associated signal generator, thereby developing a voltage greater than a predetermined value. The associated signal generator is then activated by the flow of supervisory power, whereby a signal current of preselected character is superimposed on the supervisory current. The control station registers the occurrence of the event by noting the presence of the superimposed signal current, and identifies the particular detector circuit by identifying the preselected characteristic of the signal being generated. Additionally, the routing of the supervisory current through the signal generator may be made to cause a significant decrease in the magnitude of this current. Concurrent registration of the occurrence of an event or the indication of a line fault may be noted at the control station by sensing the magnitude of the supervisory current and responding to a change of predetermined amount.

llnited States Patent 9 1 McQuown, Jr.

[451 Sept. is, 1973 SECURITY SIGNALLING WITH ALARM IDENTIFICATION [75] Inventor: Albert N. McQuown, Jr., Austin,

Tex. v

[73] Assignee: Annandale, Inc., Austin, Tex.

[22] Filed: Dec. 14, 1971 [21] Appl. No.: 207,908

[52] US. Cl. 340/164 R, 340/416 [51] Int. Cl. G08b H08 [58] Field of Search 340/164 R, 416, 280, 340/216, 276, 310

3,626,316 12/1971 Connell, .lr. 340/416 3,613,093 10/1971 Reynolds et a1. 340/416 3,550,111 12/1970 Ervin 340/416 3,069,673 12/1962 Ward et al..... 340/416 Primary Examiner-John W. Caldwell Assistant Examiner-Marshall M. Curtis Att0rneyDonald l-l. Fidler et al.

[ ABSTRACT A security system is provided which includes a central monitoring system interconnected by a two-wire telephone circuit to one or more protection loops, each being composed of one or more series-connected reporting devices or stations. A DC power supply is located in the control station for generating a supervisory current flow between the control station and the protection loop, and each reporting device is composed of a signal generator in parallel with a low resistance shunt arranged to be broken by the detected occurrence of an event sought to be observed. Detection of the event interrupts the shunt to route the supervisory current through its associated signal generator, thereby developing a voltage greater than a predetermined value. The associated signal generator is then activated by the flow of supervisory power, whereby a signal current of preselected character is superimposed on the supervisory current. The control station registers the occurrence of the event by noting the presence of the superimposed signal current, and identifies the particular detector circuit by identifying the preselected characteristic of the signal being generated. Additionally, the routing of the supervisory current through the signal generator may be made to cause a significant decrease in the magnitude of this current. Concurrent registration of the occurrence of an event or the indication of a line fault may be noted at the control station by sensing the magnitude of the supervisory current and responding to a change of predetermined amount.

22 Claims, 5 Drawing Figures PATENTED 3,760,359

sum 1 ur 4 Tmm fim l cuRRENT SENSOR LINE 1 |l ALARM 14 cOuPLER l T l I I g S J2 FREQUENCY SUPPLY IDENTIFIER I l l I cENTRAL REcE/v/NG STAT/0N 6 3A 5 1 s/GNAL OETEcTOR l GENERATOR LOOP #7 l 1 V 30 7 8 l I I SIGNAL OETEcTOR GENERATOR LOOP 2 30 9 10 1 I SIGNAL DETECTOR l 4 I GENERATOR LOOP 3 P 1 3B l L EMOTE PROTEcT/ON STAT/0N PMENTEU 8 --v- PO WE R DE LA Y REGULATOR CIRCUIT RE 615 TRAT/ON DE TE C TOR DIS PLAY IND/ CA TOR SECURITY SIGNALLING WITH ALARM IDENTIFICATION BACKGROUND This invention relates to methods and apparatus for announcing the occurrence of an event at a remote location, and more particularly relates to improved security methods and apparatus for identifying the particular situs or character of the occurring event.

It is well known to protect the security of a preselected location or area by means of electrical detectors and the like, and to interconnect such detectors with a central receiving station, whereby activation of a particular detector can be observed and answered. It is also well known to employ pre-existing facilities such as telephone lines, for the purpose of intercommunication between the central receiving station and the detectors at one or more remote locations or areas sought to be protected.

Using telephone lines for these purposes has the obvious advantage of being readily available and perhaps more economical than construction of a special communication system, especially when the area or structure sought to be protected is located a significant distance from the central receiving station. On the other hand, use of the pre-existing telephone system has several design features which must be considered if the system is to be used. First, the security systemmust be interconnected so that it does not interfere with the traffic normally expected to be transmitted by way of the telephone system or that it uses separate circuits dedicated to the exclusive use of the security system. Second, security systems, whether their purpose be fire or anti-trespass alarm, may employ a plurality of sensors or detectors located at different alarm stations. Accordingly, provision to identify the particular alarm station within any telephone circuit which has become activated is most desirable. Finally, and probably most important for trespass alarms, provision must be made to forestall any attempt to bypass or nullify the operation of the system.

Various security systems and techniques utilizing pre-existing conventional telephone lines have been proposed and adopted in many locations. In many of the more popular of these, a DCpower source is located at the central office for transmitting a predetermined current of constant magnitude over the telephone lines to and through detectors which may be composed of resistance loops of preselected value. Each resistance loop is arranged to be interrupted or broken by the occurrence of the event sought to be announced, as, for example, the opening or closing of a door or window, whereby a change in the magnitude of the line current is effected which can be observed at the central receiving station. Furthermore, any attempt to shunt around the resistance loop before it is broken will likewise produce an obserable change in the line current.

Although a system or technique of this type is presently employed in many locations, it can, nevertheless, be bridged by knowledgeable. persons. Additionally, a principal disadvantage of this type of system arises out of the fact that it cannot accommodate and thereby ignore a line disturbance resulting from a circuit fault of telephonic origin, and that the announcement which is received at the central station does not include any in dication as to which detector is involved.

Because of the obvious advantages of using a preexisting or conventional telephone system to transmit an alarm to the central receiving station, many attempts have been made to modify the aforementionedv standard system to eliminate these disadvantages. In particular, the DC power supply may be relocated to the remote location sought to be protected, and provision made to announce an event by reversing the polarity of the current being transmitted to the central receiving station. Such a modification is effective for rendering the operation of the security system independent of most types of circuit faults of telephone system origin, but it does not effect identification of one announcing detector in a multiple detector system. Furthermore, there is an obvious maintenance problem in positioning DC power sources at a multiplicity of locations sought to be protected.

Systems of the foregoing type are, of course, limited to a single protection loop for each telephone circuit. Accordingly, another modification has been employed whereby eachdetector loop or circuit is individually powered and provided with a transmitter with a normally closedswitching circuit which is connected in series with the other transmitter circuits in the overall protection loop. Activation (or deactivation as the case may be) of a particular detector will cause the transmitter associated with that detector to interrupt the loop current in acoded sequence indicative of the identity of the particulardetector. I

In another modification disclosed in U. S. Pat. No. 3,299,211, a system employing a telephone line is provided with a tonegenerator at each detector station, and is further provided with a relay switching circuit whereby actuation of a detector will cause the output of the tone generator associated therewith to be connected to the telephone line. Accordingly, each tone generator produces a different output frequency in order that the announcing detector station can be identified.

Although the system disclosed in U. S. Pat. No. 3,299,211 provides certain advantages including a priority determining scheme, it is also subject to the distinct disadvantage that because of. the priority scheme it will accommodate only one detector announcement at a time on a given telephone circuit. Furthermore, it will be noted that the system does not provide for maintenance of a supervisory current flow between the protection loop and the central receiving station, nor is the detection loop an integral part of the telephone circuit. These features seriously limit the usefulness of this system for the purpose of high security anti-trespass protection.

Another security system which has been proposed is illustrated in U. S. Pat. No. 3,599,206. This system provides for the protection of an alarm sounding circuit and includes provision for generating a high frequency AC signal and sensing the current of this signal through the supervised alarm lines and a low impedance resonance path at the remote end of the circuit. Thus, the normal inductance in the bell circuit allows only a small portion of the AC supervisory current to flow in the individual bell or alarm circuits, with the majority of the current flowing to the end of the line and through the tuned circuit. A break in the line will cause the current to decrease to the small portion flowing through the bell or horn circuits, thus indicating an interruption of service.

This system also is subject to certain limitations restricting its usefulness for general purposes. Since the low impedance resonant path is located at the end of the line, open circuits in the individual bell circuits do not provide an alarm. Furthermore, it is not practicable to partition for localizing or identifying a particular alarm situs.

Referring to U. S. Pat. No. 3,598,917, there may be seen a security system employing a continuously operating AC signal source to transmit a continuous tone signal over a telephone circuit from the protection loop to the central receiving location. Filters at the remote location (and at the telephone exchange) prevent this tone from interferring with normal telephone communication, and the occurrence of an event of interest is announced by the absence of the tone at the central receiving station. Additional equipment at the central receiving station identifies the telephone line in question, but not a particular detector location which announced the event.

Referring to U. S. Pat. No. 3,435,145, there may be seen a security system operating on the basis of announcing the event of interest by producing a measured change in DC current flow. In particular, the remote protection loop is composed of a plurality of seriesconnected resistors with shunting protection loops or switches. An open circuit in any sub-loop will, of course, produce a fixed drop in line current and it is sought to measure the magnitude of the drop to determine the particular sub-loop involved. Such a system is subject to the precise magnitude of the current drop. Since the transmission line interconnecting the protection loop with the central receiving location may itself produce significant current variation, this type of system is limited to only those applications wherein the monitoring station can be located relatively close to the area or structure sought to be protected.

Referring now to U. S. Pat. No. 3,484,533, there may be seen a special purpose system wherein an event of interest is announced by initiating a dial connect path with an impedance different from that of a telephone set, and which can therefore be identified by the telephone central office equipment. This is a type of twoparty system, which does not attempt to provide the security of a supervised line or the ability to identify a particular detector.

In U. S. Pat. No. 3,404,393, there is described a security systemwhich employs the type of pre-existing intercom system and bell wires often found in apartment houses and the like. A pulse position or time code pulse modulation technique is used to multiplex these signals on the wires of the pre-existing system. Such a system is intended to be compatible with the pre-existing system and is not applicable to an installation intended to employ circuits using a supervisory current as hereinbefore explained.

In U. S. Pat. No. 2,661,394, there is illustrated a security system with an automatic dialer and phonograph, which is activated in response to the occurrence of the event of interest, and which dials a preselected telephone number and delivers a pre-recorded message. Since such a system utilizes the regular switching circuits of the telephone system, it must include provisions for operating with these switched circuits, and thus it does not provide the security of a supervised line.

Other security systems and techniques may be found in U. S. Pat. Nos. 2,887,535; No. 2,882,423 and No. 3,559,194. However, none of these systems appears to be useful for general purposes.

These advantages of the prior art are overcome with the present invention, and novel means and methods are herewith provided for employing a pre-existing dedicated line telephone system or the like to communicate to a central receiving or monitoring station the occurrence of a preselected event at a remote protection loop composed of two or more detector stations and to further identify the particular detector station involved.

SUMMARY OF INVENTION In a preferred embodiment of the present invention, a central monitoring station is interconnected by a conventional telephone circuit or the like to one or more remotely located protection loops each composed of one or more series-connected transmitters. A source of DC current is located at the central station and is interconnected through a line coupler unit to the telephone circuit and protection loop, and' through a lowresistance shunting across each transmitter.

Each low-resistance shunt provides a bypass around its corresponding transmitter. If a shunt is broken as a result of the occurrence of the event sought to be announced, the DC current in the protection loop will necessarily be re-routed through the actuation part of the transmitter associated therewith. Since this may act to decrease the current in the protection loop, the breaking of the shunt may be evidenced at the control station by the resulting decrease in the magnitude of the DC current flowing between the protection loop and the central station. After a preselected delay interval adequate to note the decrease in DC current, the particular transmitter reacts to the DC input current now routed to it to modulate the DC line current in a unique manner identifying the detector station corresponding to the location where the event has occurred.

The subject system will desirably include a line interface circuit at the central station for coupling the modulating output from the transmitter in the protection loop to a suitable signal determining device. This line interface circuit is also preferably adapted to indicate the presence of the modulating signal on the telephone circuit to thereby provide an indication of the presence of the occurrence of the event sought to be announced. If DC current sensing circuitry is used for concurrently noting the occurrence of an event, the indication from the modulating signal will occur after the aforesaid delay interval.

Considering a preferred identifying signal to be a symmetrical repetitive waveform such as a sine wave or a square wave having a predetermined characteristic frequency, a suitable frequency determining circuit is preferably included in the central monitoring station, which may include an array of separate bandpass filters and detector circuits, each tuned to accept and register only the frequency generated by a corresponding one of the various transmitters in each protection loop. Switching means are preferably included for applying the incoming AC modulating signal from an indicating protection loop to the various bandpass filter inputs simultaneously. I

An additional feature of the invention includes provision for an aural or audible alarm which is responsive to the incoming modulating signal and which is espe-- cially useful for installations wherein the subject invention is used for monitoring a plurality of protection loops. Since the audible alarm responds to a signal on any protection loop, activation of two or more transmitters will cause the pitch of the activated aural alarm to change, thereby providing an immediate indication of the occurrence of more than one simultaneous events sought to be announced.

These and other features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.

IN THE DRAWINGS FIG. 1 is a simplified functional diagram illustrating the major components of one embodiment of a security-type system embodying the concepts of the present invention and comprising a central monitoring station interconnected by a telephone-type two-wire transmission line to one or more protection loops, each composed of one or more reporting stations or detector loops.

FIG. 2 is a detailed schematic diagram of the embodiment of the system depicted in FIG. 1, more particularly depicting the details of a control station suitable for the security system depicted in FIG. 1.

FIG. 3 is a detailed schematic diagram of a detector station suitable for inclusion in the security system depicted in FIG. I.

FIG. 4 is-a'simplified schematic representation of a modified form of a portion of the circuitry in FIG. 3.

FIG. 5 is a simplified schematic representation of another form of detector station suitable for use with the circuitry depicted in FIGS. 2 and 4.

DETAILED DESCRIPTION Referring now to FIG. 1, there may be seen a simplified diagram of a security system embodying and utilizing the concept of the present invention, being composed of a central monitoring station 2 interconnected by a two-conductor signal transmission system 3 to at least one protection loop 4 of the type basically depicted therein. As used herein, the term protection loop will be understood by those experienced in this technology to encompass all of the various reporting devices, sensors and other circuits utilized in an integral installation for securing a designated area or structure which will have at least one sensitive location, such as a door or window or the like. Accordingly, in a system of the type contemplated herein, each such location will preferably be equipped with one or more sensors which are responsive to or actuated by the occurrence of a predetermined event such as movement of the preselected door or window, etc. Therefore, a single protection loop 4 will usually be composed of a plurality of reporting circuits each individually actuable to announce the occurrence of a predetermined event.

Referring again to FIG. I, it will be seen that protection loop 4 suggested therein may have three individual reporting circuits which are each separately actuable in response to. the occurrence of a different event. More particularly, the first reporting circuit is composed of a signal generator 5 and glow-resistance shunt 6. The second reporting circuit includes another signal generator 7 and low-resistance shunt 8, and the third reporting circuit also includes a signal generator!) and a third low-resistance shunt 10. Each of the three signal generators 5, 7 and 9 may substantially be alike, but it is a feature of the presentwinvention that, when activated, each is adapted to generate a distinctively different preselected frequency.

Each of the three generators 5, 7 and 9 is coupled into the protection loop 4 in series with each other. In such an arrangement, the conductor 3A in the telephone line 3 may, for example, be conducted to the positive input terminal of the first generator 5, whereas the other conductor 38 in the telephone line 3 will be coupled to the negative input terminal of the third or last generator 9 in the protection loop 4. Accordingly, the circuit will be completed by a conductor 3C which couples the negative input terminal of the first generator 5 to the positive input terminal of the second generator 7, and by another conductor 3D which couples the negative input terminal of the second generator 7 to the positive input terminal of the third or last generator 9.

As will hereinafter be explained in detail, each of the three signal generators 5, 7 and 9 is adapted to be energized by a supervisory DC current which is supplied to the telephone line 3 by a suitable DC power supply 12 which is preferably located in the central monitoring station 2. Each of the generators 5, 7 and 9 is preferably actuable only when a predetermined event occurs, however, and thus each reporting circuit is also provided with a low-resistance shunt which is connected to effect a current bypass. Accordingly, the shunt 6 bypasses the first signal generator 5 to keep it silent or non-operative; the shunt 8 bypasses or shorts out the input of the second generator 7, and the third or last signal generator 9 is similarly bypassed or shorted out by the shunt 10.

As will also hereinafter be explained in detail, each of the three shunts 6, 8 and 10 may be any of several types of components or circuits of conventional design, such as a relay, a switch, or merely an easily frangible conductor such as a metallic strip of tape or the like. However, it is a feature of the invention that each of the shunts 6, 8 and it) constitutes or includes means for establishing the aforementioned supervisory current in the telephone line 3 at a preselected magnitude. Since the input circuits of each of the signal generators 5, 7 and 9 are adapted to provide a much higher resistance than any Lof the shunts 6, 8 and 10, substitution of any one of the generators 5, 7 and 9 for any of the shunts 6, 8 and 16 will effect a change of significant magnitude in the DC current in the telePhone line 3. in other words, if the first shunt 6 is disconnected or broken (as by the opening of a door or window or the like), this will re-route the DC current heretofore flowing from the first conductor 3A to the second conductor 38 by way of the first signal generator 5 and through the shuts 8 and MLThis produces a substantial change in the total impedance of the protection loop 4 and alters the magnitude of the DC current.

Referring now to the central monitoring station 2 suggested in FIG. 1, it will be seen that a line coupler circuit 13 and current sensor circuit ill are preferably included as a part of the circuit which carries the supervisory DC current. Accordingly, the current sensor circuit 11 preferably includes provision for responding to a change of this sort in the DC current, to register or otherwise provide anindication of the occurrence of one of the various events sought to be announced by the protection loop 4.

The change which is effected in the magnitude of the supervisory DC current will only indicate that one of a plurality of different events has occurred. When the shunt 6 is broken or otherwise disconnected, however, this activates the first signal generator as well. Thus, the first signal generator 5 will now superimpose a frequency on the DC current being carried to the control station 2 by the telephone line 3. Accordingly, the central monitoring station 2 will preferably include a frequency identifier circuit 15 of suitable design for indicating which of the three signal generators 5, 7 and 9 has been activated. In addition, means such as a variable line resistance 14 is preferably included at the central monitorlng station 2 for establishing the supervisory DC current at a suitable preselected magnitude.

Referring now to FIG. 2, there may be seen a schematic diagram of an exemplary form of the security system depicted more generally in FIG. 1 and illustrating in particular detail the operational features of the central monitoring system 2. In particular, the aforementioned DC power supply 12 may include a battery 102 having one side connected to ground or reference voltage at an output terminal 106, and having its other side coupled to another terminal 107 by way of a suitable ammeter 103, current sensor 104 and the aforementioned variable line resistance 14 depicted in FIG. 1.

Referring again to FIG. 2, it may be seen that the line coupler circuit 13 is composed of a series reset switch 150 and resistor 121 coupled in series between terminals 110 and 112, a limiting amplifier 123 having its output coupled through resistors 124 and 126 to one side of two neon lamps 125 and 128 and its two differential inputs coupled across the current sensing resistor 121 through DC blocking capacitor 122. The other sides of the two neon lamps 125 and 128 are coupled in parallel between terminals 109 and 111. A linear amplifier 135 has its output coupled to one terminal of a double pole-single throw switch 136. The switch 136 has its other terminal connected to the circuit ground as provided by the conductor between terminals 109 and 111 and its pole terminals coupled to cnductors leading into the frequency identifier circuit 15. The two differential inputs of the linear amplifier 135 are also coupled across the current sensing resistor 121 and through the DC blocking capacitor 134.

Referring again to the second neon lamp 128, it will be seen that this unit is a component of an optical relay 127 having a photo-conductive element 129 interconnected across a pair of inputs to the audible alarm circuit 311. Both the photo-conductive element 129 and the two poles of the switch 136 may also be connected to lines running to other line coupler circuits (not depicted).

As will hereinafter be explained in detail, the purpose of the audible alarm circuit 311 is to provide a concurrent audible indication of the occurrence of any significant AC signal current flowing in the wires 3A and 3B of the telephone line 3. Accordingly, it will be seen that the input terminals which are coupled across the photoconductive element 129 are connected in turn to the inputs of a variable frequency oscillator 131 having its output coupled through a power amplifier 132 to a conventional speaker or horn 133.

Referring again to the line coupler circuit 13, it may be seen that terminals 111 and 112 are interconnected by way of the telephone line 3 to a protection loop 100 having typically four or more different reporting stations each being composed of a signal generator and a low-resistance shunting means of one type or another. The four signal generator 113-116 which are connected in series between the wires 3A and 3B of the telephone line 3 may also be of any suitable design except that, when activated, each is preferably adjusted to produce an output signal which is distinctively different in some characteristic feature. As indicated, each of the four generators 113-116 is also provided with a low resistance shunting circuit which, however, will be of a type determined by the type of event sought to be detected and announced. For example, the shunt 117 across the first generator 113 is depicted in FIG. 2 as being a frangible metallic strip of the kind conventionally installed to detect the breakage of a glass panel or the like. The second shunt 118 is illustrated in FIG. 2 as taking the form of a normally closed switch or the like, which is opened by the movement of a door or window, for example.

The third shunt 119 is illustrated as a fuse or other element of the type which is responsive to abnormal temperature and which may, therefore, be employed to detect the occurrence of fire. The fourth shunt 120 illustrated in FIG. 2 maybe a photoelectric cell which is activated and rendered conductive by an impinging beam of light (not depicted) and which is rendered non-conductive by the interruption of the light beam by a trespasser or the like. In addition, it may be desirable in certain installations to selectively by-pass specific reporting stations by including one or more on-off switches 180-183 at some convenient location in the protection loop 100, such as across the input terminals of the first AC signal generator 113.

Referring now to the frequency identifier circuit 15, it may be seen that the two input terminals are coupled through a pair of capacitors 138 and 139 to the inputs of an amplifier 140 having its output coupled to four separate signal registration channels, each preferably composed of a suitable bandpass filter, signal detector, and indicator lamp, and each being tuned to respond to a different one of the various frequencies intended to be superimposed on the supervisory DC current in the telephone line 3. Thus, the bandpass filter 141 will pass only the frequency f which is generated by the first signal generator, and thus it is only the signal detector and lamp 149 which are activated when the shunt 117 is broken or otherwise disconnected. The second bandpass filter 142 passes only a frequency corresponding to the signal f provided by the second signal generator 114, and the third and fourth bandpass filters 143 and 144 pass only frequencies respectively corresponding to the signals 3 and j}, which are generated only by the third and fourth signal generators 115 and 116, respectively.

It will be seen that although all superimposed frequencies received from the protection loop 100 and applied to the amplifier 140 will be applied to the inputs of all four filters 141-144, each filter will refuse all inputs except the particular frequency to which it is tuned. Accordingly, the second signal detector 146 and lamp 150 will be activated only when the shunt 118 is broken, the third detector 147 and lamp 151 will be activated only by disconnection of the third shunt 119, and the fourth detector 148 and lamp 152 will be activated only by disconnection of the fourth shunt 120.

Referring again to FIG. 2, it will be noted that power for the security system is provided in the form of a DC current from the DC power supply circuit 12, and thus an alternative to the battery 102 may be a conventional rectifier and smoothing circuit (not depicted). This DC current may further be seen to pass through the line coupler circuit 13 which functions to sense any periodic or AC change in the signal. Thus, if any of the shunts 117-120 are broken or otherwise disconnected, the resulting frequency which is superimposed on the DC current in the telephone line 3 can be coupled off of the resistor 121 by means of the capacitor 122 and may be applied to the limiting amplifier 123. Any suitable coupling means may be employed for this purpose, of course, such as a transformer instead of the capacitor 122 illustrated in FIG. 2.

It should be noted that the supervisory power is not necessarily limited to DC current as hereinbefore described, and AC power at a standard frequency such as 60l-Iz may be used if suitable frequency rejection filters (not depicted) are installed as part of the signal path to the amplifiers 123 and 135, and if power rectifier circuits are made a part of the signal generators 113, 114, 115 and 116.

The output of the limiting amplifier 123 functions to apply sufficient voltage to the first neon lamp 125 to cause it to be illumined for the purpose of providing the initial indication of the occurrence of an event of interest in the protection loop 100. The function of the resistor 124, of course, is to limit the current in the lamp 125.

As may further be seen in FIG. 2, the output of the limiting amplifier 123 is also applied through a current limiting resistor 126 to the neon lamp 128 in the optical relay 127. Illumination of the lamp 128, of course, changes the resistance provided by the photoconductive element 129, to provide an actuating signal to the input side of the variable frequency oscillator 131 in the audible alarm circuit 311. This, in turn, produces an output from the amplifier 132 to cause the horn 133, in turn, to produce an audible alert signal.

It will be apparent to those of ordinary skill in this art that the frequency of oscillator 131 and thus the pitch of the audible signal from the horn 133 is a function of the magnitude of current flow through the photoconductive element 129 in the optical relay 127. As suggested but not specifically depicted in FIG. 2, other line coupler circuits with optical relays or the like may also be connected to the variable frequency oscillator 131, and their photo-conductive elements (not shown) will be coupled in parallel with each other as well as with the photo-conductive element 129 depicted in FIG. 2. Hence, if more than one such line coupler responds to a signal at the same time, the pitch of'the audible signal provided by the horn 133 will furnish a simple but immediate indication of cummulative occurrence of alarms from a plurality of protection loops.

When an event of interest has apparently occurred, as indicated by activation of the horn 133 and illumination of the lamp 125, the person monitoring the system may close the monitor switch 136 to couple the superimpoed signal component to the inputs of the four bandpass filters 141-144. Any DC bias component of the incoming signal is, of course, blocked by the capacitors 134, 138 and 139, so that the amplifier 140 will receive and condition only the signal of interest.

Referring now to FIG. 3, there may be seen a schematic diagram of an exemplary embodiment of a signal generator and shunting circuit suitable for the purposes of a security system such as that depicted in FIGS. 1 and 2. More particularly, the circuitry illustrated in FIG. 3 may be employed to correspond to the second signal generator 114 and shunt 118 employed in the protection loopdepicted in FIG. 2.

Thus, the signal generator 114 may be composed of a power regulator section, a delay circuit, an oscillator section, a signal modulator section, and a latch section. As indicated, the power section is composed of a pair of zener diodes 159 and 160 and a load resistor 158 arranged in series between the two input terminals 151 the diode 152, and a capacitor 161 connected in parallel with thediode 160. The delay circuit portion is composed of a capacitor 162 connected between the junction between the two diodes 159 and 160 and one end of a resistor 163 having its other end coupled to the base electrode of a transistor 164.

The oscillator section, which is of conventional multivibrator design, is composed of a pair of transistors 165 and 166 arranged oppositely of each other with their emitter electrodes coupled to terminal 152, a capacitor 168 coupled between the base electrode of transistor 166 and the collector electrode of transistor 165, another capacitor 167 similarly coupled between the base electrode of transistor 165 and the collector electrode of transistor 166,. bias resistors 169 and 170 each coupled to a base electrode of one of the two transistors 165 and 166, and load resistors 171 and 170 each coupled to a collector electrode of one of the two transistors 165 and 166. The opposite ends of each of these resistors 169-172 are each coupled to the junction between the diodes 159 and 160 in the power regulator section and to the opposite side of the capacitor 162 in the delay circuit portion.

The modulator section may be seen to be composed of a transistor 174 which also has its emitter electrode coupled to the input terminal 152, and which has its collector electrode coupled through a load resistor 175 to the other input terminal 151. The base electrode of the transistor 174 is preferably coupled through another resistor 173 to the collector electrode of the second stage transistor 166 in the oscillator section.

The latch section is preferably composed of a transistor 177 which also has its emitter electrode coupled to the input terminal 152 and its base electrode coupled through a load resistor 176 to the junction between the two diodes 159 and 160 in the power regulator section. The collector electrode is coupled through a biasing resistor 178 to the bypass or shunt terminal 154, the other shunt terminal 153 being coupled directly to the input terminal 151 in the power regulator section. A silicon controlled rectifier 157 is also preferably coupled between the shunt terminal 154 and the input terminal 152.

Referring now to the circuitry of the shunt 1 18, there may be seen that a suitable conductive circuit having a relatively small resistance as depicted by resistor 155 is coupled directly between the shunt terminal 153 and one contact of a suitable switch 156. The other contact of the switch 156 is coupled to the other shunt terminal During normal operation of the security system, the switch 156 will be closed, and since the resistance value of the shunt conductor 155 is relatively quite small, the voltage drop across the terminals 151 and 152 is only a very small portion of the available supply voltage. Accordingly, the voltage across the two input terminals 151 and 152 is substantially below the breakdown voltage characteristic of the zener diode 159.

If the switch 156 is opened, however, this will remove the short circuit normally coupled across the input terminals 151 and 152, and, since the available DC voltage at the terminals 151 and 152 is substantially greater than the breakdown voltage characteristic of the zener diode 159, this will produce a flow of DC current through the resistor 158 and diode 159 to the capacitor 161. When the capacitor 161 is charged to the level of the zener voltage of the diode 160, this will produce a stable operating voltage for the remainder of the signal generator 114. Accordingly, the values of the resistor 158 and the diodes 159 and 160 will preferably be selected to produce a reduction in the DC current flowing in the telephone line 3 which is sufficient to trigger the current sensor 104 in the DC power supply circuit 12. When an adequate operating voltage is initially developed across the filter capacitor 161 and diode 160, this change in voltage will be coupled by the capacitor 162 and resistor 163 to the base electrode of the transistor 164, to cause the transistor 164 to momentarily clamp the operation of the oscillator and modulator sections.

In the oscillator section, the frequency of its output signal is determined by the RC time constant which, in

turn, is determined by the values of the resistors 169 and 170 and the capacitors 167 and 168. This output signal is coupled through resistor 173 to transistor 174 which acts to switch the resistor 175 across the terminals 151 and 152. The value of the resistor 175 is preferably such that it produces a significant decrease in the impedance between the two terminals 151 and 152 to produce a correspondingly significant increase in the supervisory DC current in the telephone line 3 according to the frequency and waveform characteristic of the oscillation section. Since the average value of the modulated DC current is greater than its minimum value, however, the delay circuit functions to hold back the oscillator and modulator sections for a predetermined time interval sufficient to insure activation of the DC current sensor 104.

It will be apparent to those with experience in the use of security systems of an anti-trepass type that it is extremely iinportant for the alarm to continue even though the reacting detector element be immediately restored to a normal condition. In other words, if a protected door (for example) is moved sufficient to open the switch 156, it is essential that the security system remain in its alarm state, notwithstanding that the door is instantly reclosed. Accordingly, the purpose of the latch section is to keep the oscillator section in an activated condition, notwithstanding that the switch 156 is reclosed and the shunt 118 is restored across the terminals 151 and 152.

Referring again to FIG. 3, it will be seen that when DC current flow through the shunt circuit 118 is either interrupted completely or drops to some preselected minimum level, this will cause the regenerative action of the silicon controlled rectifier 157 to cease. The appearance of a voltage across the capacitor 161 will produce a current flow in the resistor 176 which turns on the transistor 177, and this, in turn, effectively shorts out the trigger gate of the silicon controlled rectifier 157. If the swich 156 is thereafter closed, the input potential of terminals 151 and 153 will be reconnected to terminal 154, and thus to the resistor 178 and the silicon controlled rectifier 157. The transistor 177 is now conducting, however, and thus the current in the resistor 178 is now diverted from the trigger gate of the silicon controlled rectifier 157. Accordingly, the silicon controlled rectifier 157 will not now fire to complete the bypass circuit between terminal 154 and terminal 152.

It is necessary in any security system, of course, to provide some kind of reset means for restoring the system to its normal operating condition. In the system depicted in FIG. 2, this can be achieved by momentarily removing the voltage coupled across the terminals 151 and 152 of the activated signal generator 114 depicted in FIGS. 2 and 3, since loss of voltage at the two terminals 151 and 152 will discharge the capacitor 161, and.

this, in turn, removes the activating voltage across resistor 176 and the current from the base electrode of the transistor 177.

After the transistor 177 in the latching circuit has been turned off, the input potential may be restored to the terminals 151 and 152, and if the switch 156 has been previously restored to its normally closed condition, the DC current through the terminals 151 and 152 will again be routed through the shunt 118 to bypass the signal generator 114 as hereinbefore explained. If the switch 156 has not previously been reclosed, however, the generator 114 will again become activated to announce the fact.

Once current flow through the shunt 118 has been restored, current will again flow through the resistor 178 to the gate electrode of the silicon controlled rectifier 157. The transistor 177 is delayed in its operation by the action of the resistor 158 and capacitor 161, so that the silicon controlled rectifier 157 will regain conductivity and thereby complete the bypass. Once the rectifier fires, of course, the completed bypass or short circuit across the terminals 151 and 152 will prevent any current from flowing through the diode 159 to the capacitor 161.

The reset function may be performed by the switch 150, which is located at the central monitoring station 2 and preferably between the terminal 112 and the resistor 121 of the line coupler circuit 13 to interrupt the flow of current to the protection loop. Alternatively, this reset function may be performed by a switch 181 located in the protection loop across the terminals 151 and 152 of the signal generator 114 in FIG. 3.

In those instances where the depicted security system is employed for fire alarm purposes, it is acceptable for purposes of convenience to locate the reset means adjacent the alarm circuit which is to be restored to normal operation. Accordingly, normally open reset switches 180-183 may be located across the input terminals of each of the four signal generators 113-116, as indicated in FIG. 2. It will be apparent that any one of these four reset switches 180-183 may be used, of course, to not only reset the latch circuit but to bypass the action of the signal generator and thus the function of the reporting station.

In those instances where the security system is intended to be used for anti-trespass purposes, it is undesirable to locate the reset means any place where it might be available to unauthorized personnel. Accordingly, it is preferable for such purposes to use only the reset switch 150 in the central monitoring station 2 and to make switches180-l83 lock operated for bypassing specific stations in the protection loop 100.

The design of the oscillator portion of the signal generator 114 depicted in FIG. 3 is suitable for the purposes of a security system of the type depicted in FIGS. 1 and 2, however, alternative designs may be employed, such as those employing RC phase shift or tuned LC oscillator stages, or the like.

As hereinbefore suggested, the signal generators 5, 7 and 9 depicted in FIG. 1 may be adapted to provide identification insome form other than a characteristic frequency. For example, provision may be made instead to generate pulses according to a preselected time code, and in such a case, the so-called frequency identifier circuit must be replaced with some identification means which is responsive to such an identification signal.

Referring now to FIG. 4,there may be seen a simplified schematic representation of an exemplary form of a signal generator suitable to replace the circuitry depicted in FIG. 3, and adapted to produce an identity signal having the form of a preselected time code or the like. In particular, it will be noted that the occurrence of the regulated DC voltage across zener diode 160 and filter capacitor 161 provides the power for the pulse generating circuits of FIG. 4. A clock circuit 201 is used to generate a uniform train of pulses 202 at a predetermined rate, the-time interval between each pulse representing a characteristic time slot. The clock circuit 201 may be any one of several designs known in the art such as a multivibrator similar to the oscillator of FIG. 3 or a unijunction transistor oscillator. The uniform train of pulses 202 is applied to a binary counting circuit 203 consisting of a plurality of series connected flip-flop circuits. Such a counting circuit is well known in the art and is available as an integrated semiconductor circuit. The output of the depicted four stage counter is a four wire binary number 204 representative of the cummulative number of clock pulses with the count repeating each 16 pulses.

The output of the four binarydigit counter 204 is then connected to a binary to decimal decode circuit 205 having ten transistor output connections 206. Such a decode circuit is also available as an integrated circuit, and functions to turn ON one of the ten output transistors in response to the binary number applied to its input. Since the counter 203 is continuously stepping through its 16 possible counts, the decode circuit 205 sequentially turns ON its ten output transistors with counts 11 through 16 producing no output.

A predetermined repetitive ten binary digit code may be generated by parallel connecting the appropriate combination of transistor outputs 206 to a common output 207. This coded output may be connected to the aforesaid load resistor 175 to modulate the current on the line. A preferred code may use time slots one, five and ten of each 10 pulse code to identify a registration position, with time sots two, three, four and six through nine used to identify one of a possible 128 remote stations, with time slots 11 through 16 used as a code block separation space.

Referring now to FIG. 5, there may be seen a simplitied schematic representation of an exemplary form of an identification circuit adapted to select and identify a signal of the type generated by the circuitry depicted in FIG. 4, and which may be substituted for the frequency identification circuit 15 depicted in detail in FIG. 2, In particular, there may be seen a decoder circuit wherein the modulation signal from the line coupler 13 is connected to a pulse amplifier 208 which serves to condition the modulation signal to a form suitable for use in the subsequent decoding integrated circuits. The received coded pulse train 209 is applied to the output of a shift register circuit 210 and to a clock pulse generator circuit 21 1. The clock circuit 21 1 oper 210 to step the incoming signal pulses 209 through the shift register 210.

The shift register 210 performs a serial-to-parallel conversion and provides a 12 wire representation of the passing coded pulse train. Decoding is accomplished by first recognizing registration of the coded block by the presence of a pulse in time slots one, five, and 10, and a lack of pulses in time slots zero (or16) and 11 by the detector 214. At the latter part of this time interval, signal pulses 215 for about four of the seven time slots or bits are recorded in a latch or memory circuit 216. At the next pass of the repetitive code block, a comparator circuit 218 first compares the binary number 217 in the latch circuit with the 'correspondingnumber 215 in the shift register. If the two binary numbers match during the first part of the time interval as determined by the registration detector 214, then a STOP command 219 is applied by the AND gate 312 to the clock circuit freezing the recognized code in the shift register. If a match between the two binary numbers 215 and 217 is not obtained, the number 215 is then recorded in the latch 216.

This decoder circuit thereby looks for two consecutive identical code blocks. If correspondence is achieved as indicated by the STOP command 219, the identified number 220 in the shift register is applied to suitable display decode circuit 221 and to a display indicator 222. Those skilled in the art will recognize that this is only one of several means for encoding and decoding adigital message and is included here to show the applicability of using an information signal having a predetermined time charateristic as well as a signl having a predetermined frequency characteristic.

Other variations and modifications will become apparent to those of ordinary skill in this technology. Accordingly, it should be clearly understood that the structures and techniques described herein and depicted in the accompanying drawings are illustrative only and are not intended as limitations on the scope of the present invention.

' What is claimed is:

1. An electrical security system or. the like, comprisa source of power,

transmission means interconnected with said source of power for conducting a current flow,

a normally closed detecting circuit interconnected with said transmission means having an impedance selected to maintain'a voltage lower than a predetermined threshold value and openable upon the occurrence of a preselected event, and

an identification circuit means connected in parallel with said detecting circuit and including means responsive to an increase in said voltage above said threshold value for superposing on said current flow an information signal representative of said preselected event.

2. The signal generator described in claim 1, further including modulating means for superimposing said signal on said current flow in said transmission means upon the occurrence of said predetermined event.

3. The signal generator described in claim 2, further including latching means for maintaining activation of said signal generator upon subsequent reclosure of said detecting circuit.

4. The system described in claim 3, further including first sensing means for deriving a first indication in response to the occurrence of said superimposed signal in said transmission system.

5. The system described in claim 4, further including amplifying means for deriving a signal voltage functionally corresponding to said superimposed signal current in said transmission means.

6. The system described in claim 5, further including a signal determining means responsive to an information signal having characteristics substantially corresponding to the characteristics of said superimposed signal current, and

a switching means for selectively coupling said amplifying means to said signal determining means, and

an indicating means responsive to said signal determining means for announcing the occurrence of said event.

7. A security system or the like having a central monitoring station and at least one remote station, comprising a source of DC power for generating a supervisory DC current of preselected magnitude,

a transmission means for carrying said DC current between said stations,

a first conductive path of relatively low resistance at said remote station and connected to said transmission means for conducting substantially all of said DC current flowing between said stations at said preselected magnitude,

a second path also connected to said transmission means including a signal generating means for producing a periodic signal having a preselected frequency characteristic upon interruption of said first path and routing of said DC current through said second path, and

means for superimposing said periodic signal on said supervisory DC current flowing in said transmission means.

8. The system described in claim 7 further including a detecting means responsive to the occurrence of a preselected event for interrupting at least momentarily said first conductive path.

9. The system described in claim 16 wherein said second path further includes a sensing means for determining the occurrence of an interruption in said DC current flow in said first conductive path, and

a current interrupting means responsive to said sensing means to maintain said interruption of DC current from said first path.

10. The system described in claim 9, further including at said central monitoring station a first modulated current sensing means for deriving a first indication in response to the occurrence of said superimposed periodic signal, and

an AC coupling means and amplifying means for deriving an AC voltage functionally corresponding to said superimposed periodic signal.

11. The system described in claim 18, further including resonating means responsive to only an AC input voltage having frequency characteristics substantially corresponding to the frequency characteristics of said superimposed periodic signal,

switching means for selectively coupling said amplifying means to said resonating means in response to said first indication, and

signal detection means responsive to said resonating means for indicating the identity of said event.

12. The system described in claim 19, further including a DC current sensing means for deriving a second indication in response to a change in magnitude of said supervisory DC current to less than a preselected magnitude.

13. The system described in claim 20, wherein said signal generating means includes delay means responsive to the initial flow of current in said second path for decreasing said supervisory DC current during a discrete time interval initiated by the occurrence of said event.

14. The system described in claim 10, further including voltage generating means responsive to said superimposed signal in said transmission system for generating a voltage increment of preselected magnitude,

a variable frequency oscillator for generating a tone frequency related to the magnitude of said voltage increment, and

alarm means for providing an audible tone corresponding in pitch to said tone frequency,

said oscillator also adapted to receive at least one additional voltage increment of said preselected magnitude and to generate said tone frequency as a function of the sum of said increment from said voltage generating means and said additional increment.

15. A security system or the like having a central monitoring station and at least one remote station, comprising a source of DC power for generating a supervisory DC current of preselected magnitude,

a transmission means for carrying said DC current between said stations,

a first conductive path of relatively low resistance at said remote station and connected to said transmis sion means for conducting substantially all of said DC current flowing between said stations at said preselected magnitude,

a second path also connected to said transmission means including a signal generating means for producing a code signal having a preselected timedependent characteristic upon interruption of said first path and routing of said DC current through said second path, and

means for superimposing said code signal on said supervisory DC current flowing in said transmission means.

16. The system described in claim further including a detecting means responsive to the occurrence of a preselected event for interrupting at least momentarily said first conductive path.

R7. The system described in claim 16 wherein said second path further includes a sensing means for determining the occurrence of an interruption in said DC current flow in said first conductive path, and

a current interrupting means responsive to said sensing means to maintain said interruption of DC current from said first path.

18. The system described in claim 17, further including at said central monitoring station a first modulated current sensing means for deriving a first indication in response to the occurrence of said superimposed code signal, and w an AC coupling means and amplifying means for deriving a code voltage functionally corresponding to said superimposed code signal.

19. The system described in claim 18, further includdecoding means responsive to only an input code voltage having time characteristics substantially corresponding to thetime characteristics of said superimposed code signal,

switching means for selectively coupling said amplifying means to said decoding means in response Jo said first indication, and

signal display means responsive to said decoding means for indicating the identity of said event.

20. The system described in claim 19, further including a DC current sensing means for deriving a second indication in response to a change in magnitude of said supervisory DC current to less than a preselected magnitude.

21. The system described in claim 20, further including a delay means as part of said signal generator responsive to the initial flow of current in said second path for decreasing said DC current for a discrete preselected time interval following the occurrence of said event.

22. The system described in claim 21, further including voltage generating means responsive to said superimposed signal in said transmission system for generating a voltage increment of preselected magnitude,

a variable frequency oscillator for generating a tone frequency related to the magnitude of said voltage increment, and

alarm means for providing an audible tone corresponding in pitch to said tone frequency,

said oscillator also adapted to receive at least one additional voltage increment of said preselected magnitude and to generate said tone frequency as a function of the sum of said increment from said voltage generating means and said additional increment.

[U ITED STATES PATENT OFFICE CERTIECATE CORRECTION Patent No. 3 I760 11359 Dat d Sept. l8 1973 Inventor(s) Albert MCQuOWn' 22 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 6, line "9,- "conducted" should read connected line 32, after "input insert side I line 49, "telePhone" should read telephone "in" should be In Col, 7, line l4,'"monitorIng" should read monitoring line 41, "cnductors" should read conductors 7 Col. 8, line '3' "generator" should read generators Col, 9, line .61, 'rimpoed" should read rimposed Col. 10, line 12, "the diode" should be -and-'-y; Col. 13, line'62, "s'ots" should read slots' Col. 14, line' 49,, "signl" should read signal Col, 15, line; 59, "16" should be 8 Col. 16, line"7 "18" should be 10 v line, l 8, "19" should be ll line 24, "20" should be 12 Col. 17, line 29, "J0" should be to Signed and sealed this 5th day of March 197A.

(SEAL) Attest: I I

EDWARD M.FLETCHER,JRT T c. MARSHALL DANN Attesting Offi er Commissioner of Patents O PC4050 (169) uscoMM-Dc soa mme s t U GOVERNMENT PRINTING OFFICE 3 '99 366" 33l x 

1. An electrical security system or the like, comprising a source of power, transmission means interconnected with said source of power for conducting a current flow, a normally closed detecting circuit interconnected with said transmission means having an impedance selected to maintain a voltage lower than a predetermined threshold value and openable upon the occurrence of a preselected event, and an identification circuit means connected in parallel with said detecting circuit and including means responsive to an increase in said voltage above said threshold value for superposing on said current flow an information signal representative of said preselected event.
 2. The signal generator described in claim 1, further including modulating means for superimposing said signal on said current flow in said transmission means upon the occurrence of said predetermined event.
 3. The signal generator described in claim 2, further including latching means for maintaining activation of said signal generator upon subsequent reclosure of said detecting circuit.
 4. The system described in claim 3, further including first sensing means for deriving a first indication in response to the occurrence of said superimposed signal in said transmission sysTem.
 5. The system described in claim 4, further including amplifying means for deriving a signal voltage functionally corresponding to said superimposed signal current in said transmission means.
 6. The system described in claim 5, further including a signal determining means responsive to an information signal having characteristics substantially corresponding to the characteristics of said superimposed signal current, and a switching means for selectively coupling said amplifying means to said signal determining means, and an indicating means responsive to said signal determining means for announcing the occurrence of said event.
 7. A security system or the like having a central monitoring station and at least one remote station, comprising a source of DC power for generating a supervisory DC current of preselected magnitude, a transmission means for carrying said DC current between said stations, a first conductive path of relatively low resistance at said remote station and connected to said transmission means for conducting substantially all of said DC current flowing between said stations at said preselected magnitude, a second path also connected to said transmission means including a signal generating means for producing a periodic signal having a preselected frequency characteristic upon interruption of said first path and routing of said DC current through said second path, and means for superimposing said periodic signal on said supervisory DC current flowing in said transmission means.
 8. The system described in claim 7 further including a detecting means responsive to the occurrence of a preselected event for interrupting at least momentarily said first conductive path.
 9. The system described in claim 16 wherein said second path further includes a sensing means for determining the occurrence of an interruption in said DC current flow in said first conductive path, and a current interrupting means responsive to said sensing means to maintain said interruption of DC current from said first path.
 10. The system described in claim 9, further including at said central monitoring station a first modulated current sensing means for deriving a first indication in response to the occurrence of said superimposed periodic signal, and an AC coupling means and amplifying means for deriving an AC voltage functionally corresponding to said superimposed periodic signal.
 11. The system described in claim 18, further including resonating means responsive to only an AC input voltage having frequency characteristics substantially corresponding to the frequency characteristics of said superimposed periodic signal, switching means for selectively coupling said amplifying means to said resonating means in response to said first indication, and signal detection means responsive to said resonating means for indicating the identity of said event.
 12. The system described in claim 19, further including a DC current sensing means for deriving a second indication in response to a change in magnitude of said supervisory DC current to less than a preselected magnitude.
 13. The system described in claim 20, wherein said signal generating means includes delay means responsive to the initial flow of current in said second path for decreasing said supervisory DC current during a discrete time interval initiated by the occurrence of said event.
 14. The system described in claim 10, further including voltage generating means responsive to said superimposed signal in said transmission system for generating a voltage increment of preselected magnitude, a variable frequency oscillator for generating a tone frequency related to the magnitude of said voltage increment, and alarm means for providing an audible tone corresponding in pitch to said tone frequency, said oscillator also adapted to receive at least one additional voltagE increment of said preselected magnitude and to generate said tone frequency as a function of the sum of said increment from said voltage generating means and said additional increment.
 15. A security system or the like having a central monitoring station and at least one remote station, comprising a source of DC power for generating a supervisory DC current of preselected magnitude, a transmission means for carrying said DC current between said stations, a first conductive path of relatively low resistance at said remote station and connected to said transmission means for conducting substantially all of said DC current flowing between said stations at said preselected magnitude, a second path also connected to said transmission means including a signal generating means for producing a code signal having a preselected time-dependent characteristic upon interruption of said first path and routing of said DC current through said second path, and means for superimposing said code signal on said supervisory DC current flowing in said transmission means.
 16. The system described in claim 15 further including a detecting means responsive to the occurrence of a preselected event for interrupting at least momentarily said first conductive path.
 17. The system described in claim 16 wherein said second path further includes a sensing means for determining the occurrence of an interruption in said DC current flow in said first conductive path, and a current interrupting means responsive to said sensing means to maintain said interruption of DC current from said first path.
 18. The system described in claim 17, further including at said central monitoring station a first modulated current sensing means for deriving a first indication in response to the occurrence of said superimposed code signal, and an AC coupling means and amplifying means for deriving a code voltage functionally corresponding to said superimposed code signal.
 19. The system described in claim 18, further including decoding means responsive to only an input code voltage having time characteristics substantially corresponding to the time characteristics of said superimposed code signal, switching means for selectively coupling said amplifying means to said decoding means in response to said first indication, and signal display means responsive to said decoding means for indicating the identity of said event.
 20. The system described in claim 19, further including a DC current sensing means for deriving a second indication in response to a change in magnitude of said supervisory DC current to less than a preselected magnitude.
 21. The system described in claim 20, further including a delay means as part of said signal generator responsive to the initial flow of current in said second path for decreasing said DC current for a discrete preselected time interval following the occurrence of said event.
 22. The system described in claim 21, further including voltage generating means responsive to said superimposed signal in said transmission system for generating a voltage increment of preselected magnitude, a variable frequency oscillator for generating a tone frequency related to the magnitude of said voltage increment, and alarm means for providing an audible tone corresponding in pitch to said tone frequency, said oscillator also adapted to receive at least one additional voltage increment of said preselected magnitude and to generate said tone frequency as a function of the sum of said increment from said voltage generating means and said additional increment. 